This invention relates to mine roof suspension bolt anchors and may be described as an expansion anchor for a mine roof bolt. More particularly the present invention relates to mine roof bolt anchors which employ deformable transverse members to engage an anchor bolt bore hole.
Mine roof suspension bolts have generally replaced timbers as the means for supporting the roof of a mine. These bolts are anchored within bore holes drilled into the rock strata overlaying the mine. The bolt is urged tightly against a steel roof bearing plate and the stratified rock is thereby compressed and bound together into a self-supporting laminated beam that is strong enough to support material above and around the excavation.
The obvious problem with mine roof bolts is the stability of the anchor. If the anchor slips, the self-supporting beam can be weakened and sag or fall. Therefore many attempts have been made to devise anchors which are impervious to slippages. Representative of anchors which are in some manner similar in appearance to the present invention are those shown in U.S. Pat. Nos. 1,367,080; 1,352,201; 2,950,602; 4,100,748; and 4,147,458, although some of these are not suitable for use as mine roof bolt anchors. The '080 patent and '201 patent are directed to general anchoring means and, as will be seen, are not adaptable for use as mine roof anchors. The '602, '748 and '458 patents are directed specifically to mine roof supports.
The '080 patent shows a bolt anchorage which utilizes a set of flat hard metal washers which are stacked on a bolt in alternating sequence with a plurality of transversely bent washers of soft or deformable material. The washers are compressed to distend the bent washers into engagement with the bore hole walls. This compression is achieved by either: inserting the bolt head into the bore hole in a non-rotatable manner and compressing the washers by means of a nut threaded onto the bolt, thereby leaving a portion of the bolt protruding from the hole; by impaction of the washers into the hole and then threadably engaging the washers onto the bolt; or by placing a nut in the bottom of the hole in a non-rotating manner, filling the hole with washers and inserting a bolt to compress the washers. Each instance appears to require a bore hole full of washers and does not allow further travel of the bolt; furthermore, it appears that the bent washers on the ends of the alternating series would flatten first, thereby requiring motion of the flattened washer to enable flattening of the other washers. It is readily apparent that motion of a seated washer in an expansion anchor has deleterious effects.
The '201 patent shows an expansion anchor using a plurality of curved deformable washers mounted in facing pairs with the curvature of the adjacent washers being in opposition. These washers are compressed by means such as described with regard to the '080 patent. Again, it should be noted that the end washers are apparently going to engage the bore hole walls first and must slide along the hole in order to flatten the remaining washers.
The '602 patent shows an expansion anchor which uses split rings circumscribing the roof bolt. The split rings are of two different diameters and are designed to be compressed with one size within the other to engage both the bore hole and the roof bolt. A plurality of such anchor rings may be used with each set of anchor rings separated by a metal tube which transmits forces between the upper and lower rings. This device relies on an upper nut which must non-rotatably engage the bore hole through the use of outward projections and a lower nut or stop which must advance with the bolt to compress the split rings vertically and thereby cause their lateral expansion. When the rings are fully seated the bolt cannot be advanced further due to the lower stop engaging the rings.
The '748 patent shows an expansion anchor which uses a wedge and bail supported sleeve which move responsive to the rotation of a bolt such that the sleeve is compressed between the downwardly traveling wedge and the wall of the bore hole. The primary gripping force in this type anchor has been found to exist only in the region of the sleeve that is first expanded into engagement with the wall of the bore hole.
The '458 patent shows a roof pin anchor which utilizes a plurality of resilient curved locking discs which are spaced apart by intermediate reinforcing spacers therebetween. The assemblage is forced upwardly into the bore hole. The discs are not compressed and engage the walls of the bore hole solely due to weight pulling downward on the bolt. The spacers provide reinforcement for the subjacent disc and provide a locus for the pivoting motion of the periphery of the subjacent and superjacent discs. More than one spacer element may separate the discs.
Despite the achievements and efforts involved in developing these and other anchors, the problem of slippage remains. The problem may be simplistically termed a failure to provide adequate force on the bearing surfaces engaging the walls of the bore hole. As noted with respect to the '748 and '080 patents this is primarily due to the uneven engagement of the anchor mechanism with the bore hole walls which results in decreased useful bearing surface.